The control of nitrogen oxides (primarily nitric oxide, NO, and nitrogen dioxide, NO2) is an important national goal. Nitrogen oxide emissions contribute to the formation of smog, fine particulate matter (often termed PM2.5), and regional haze. Nitrogen oxides also participate in the atmospheric reactions that lead to the formation of acid rain. To control nitrogen oxides, the Clean Air Act includes numerous significant requirements that apply to stationary sources such as fossil-fuel fired boilers, wood waste fired boilers, municipal waste incinerators, medical waste incinerators, cement kilns, and other industrial processes. Industrial sources are presently applying a variety of nitrogen oxides control techniques either alone or as combinations of systems to achieve the NOx control limitations that become effective in the near future.
Selective noncatalytic reduction (SNCR) systems can be installed on a variety of fossil fuel and waste fuel fired combustion systems and on cement kilns. SNCR systems inject either ammonia or urea reagent into the combustion gas stream at a point in the combustion or kiln process where the gas temperature is in the range of 1600° F. to 2000° F. In this temperature range, the ammonia or urea reagents react with the NOx compounds, which are then chemically reduced to harmless diatomic nitrogen, N2. Due to the limitations of reagent mixing in the gas stream and the limited residence time of the gas stream in the critical temperature range, a portion of the ammonia and urea reagents fails to react. Ammonia gas is emitted from both ammonia and urea based SNCR systems. These emissions are termed ammonia “slip” and are usually in the range of 2 ppm to 40 ppm. To minimize ammonia emissions, operators of SNCR systems must often reduce the quantity of ammonia or urea reagent injected (stoichiometric ratio of reagent to NOx) into the system and, thereby, also reduce the overall extent of NOx emission reduction. The limits of NOx reduction efficiency of SNCR systems due to ammonia slip related problems are described in technical papers by Hurst & White (361), Jones (961), Quartacy et al. (250), Moilanen et al (576), Gullett et al (597), Pachy et al. (598), and Sun et al. (956).
Selective catalytic reduction (SCR) systems use a vanadium pentoxide-titanium dioxide-tungsten oxide or zeolite catalyst bed mounted in a portion of the combustion system effluent gas stream that is at temperatures of 500° F. to 1000° F. Ammonia reagent is injected into the combustion gas stream before it reaches the catalyst bed. In the presence of the catalyst, the ammonia chemically reduces the nitrogen oxides. SCR systems can achieve high NOx reduction efficiencies when stoichiometric levels of ammonia reagent are used; however, some of the ammonia can penetrate the SCR system. SCR system operators must often reduce the rate of ammonia injection in order to avoid undesirable levels of ammonia slip. This reduces the NOx control efficiency of the SCR system. The ammonia slip imposed limitation to the performance of SCR systems is described further in technical papers by Donnelly et al. (88), Durilla et al. (1170), Buschmann et al. (116), The U.S. Department of Energy (978), and Gullett (597).
In addition to SNCR systems and SCR systems, operators of fossil fuel-fired boilers, waste-fired incinerators, and cement kilns often use modified combustion system operating conditions, low NOx burners, and gas reburning systems to suppress NOx concentrations before the gas streams to be treated reach the SNCR and/or SCR equipment. When these NOx concentration suppression techniques are used to their maximum design limits, additional organic compounds are formed and remain in the gas stream. The process disclosed here benefits from the presence of these organic compounds, which serve as participants in the free radical chain reactions used to destroy ammonia gas.
Available ammonia control techniques are not well suited for the control of ammonia gas emissions from NOx control systems. Ammonia scrubbers, such as those used in some chemical industry sources, use packed bed, tray tower, and spray tower absorbers. All of these scrubbers are designed for ammonia gas concentrations substantially higher than the concentrations generated by SNCR and SCR systems operating at or near their design limits. These conventional ammonia wet scrubbers have poor efficiencies for gas streams having low ammonia gas concentrations. Furthermore, the wet scrubbers require large vessels and liquid handling systems and, thereby, cannot be retrofitted into many existing boiler stations having limited space. The liquid streams from the scrubbers must be treated to prevent contaminant releases to surface waterways or the groundwater.
Conventional ammonia scrubbers do not provide an economically feasible and practical means to control ammonia emissions from NOx systems.
Photochemical destruction of volatile organic compounds (VOCs) is known. U.S. Pat. No. 3,977,952 discloses a process for the decomposition of one or more carbon-containing compounds such as in an industrial waste or flue gas containing volatile organic compounds, oxygen, and water vapor. The method is carried out by exposing humidified gas to radiation of a wavelength of about 20 to 600 nanometers.
In some industrial processes, such as pyroprocessing of cement, recovery of the particulate matter solids produces material that is of economic importance. A discussion of dry sorption methods is found in U.S. Pat. No. 6,080,281 teaching an emission control process using photocatalytic and nonphotocatalytic aerogels for adsorption, and exposing the photocatalytic aerogel material containing adsorbed VOCs to ultraviolet (UV) radiation resulting in VOC destruction.
U.S. Pat. No. 4,210,503 discloses a direct photolysis method for controlling gaseous emissions, particularly vinyl chloride, by exposing the emissions to UV light and, thereafter, absorbing such decomposition products in a scrubber that substantially eliminates the vinyl chloride and most other decomposition products from the effluent stream.
U.S. Pat. No. 4,981,650 discloses a method to remove dioxin-contaminated waste by extraction in a liquid capable of extracting dioxins. A hydrogen donor is added to the extracting solvent or later during addition of an activating agent. The dioxin-containing liquid extract is treated in a direct photolysis reactor that contains immersion UV lamps.
U.S. Pat. No. 5,045,288 discloses the removal of halogenated and non-halogenated volatile and non-volatile organic contaminants from a gaseous stream by mixing a gaseous oxygen bearing substance with the contaminated gaseous stream, contacting the mixture with a solid photocatalyst, and exposing the photocatalyst and organic components to UV light having a wavelength up to 600 nanometers. The catalyst is pre-selected to prevent formation of a liquid phase.
U.S. Pat. No. 5,417,825 discloses a thermal photolytic process that uses high temperatures in combination with radiation exposure to induce a photochemical reaction to detoxify a wide variety of organic pollutants, for example, chlorinated aromatic hydrocarbons. The hydrocarbons are treated in the gaseous phase by heating the gas to a temperature greater than 200° C., preferably 600° C. to 800° C., and exposing the heated gas to radiation at wavelengths of less than 280 nanometers, preferably from 185 nanometers to 280 nanometers, for at least two seconds.
U.S. Pat. No. 5,650,549 teaches a photothermal process for the detoxification of chlorinated aromatic hydrocarbons contained in a gas stream. The chlorinated aromatic hydrocarbons are heated to a temperature of greater than 200° C. to form a gas stream, or a pre-existing chlorinated aromatic hydrocarbon containing gas stream is produced from a combustion source at a temperature of greater than 200° C. The gas stream is exposed to radiation at a wavelength of less than 280 nanometers for at least one second to convert the chlorinated aromatic hydrocarbons nontoxic reaction products, and the gas stream is released to the atmosphere.
U.S. Pat. No. 5,839,078 discloses a method of direct vitrification of nuclear waste comprising the steps of providing waste in the form of relatively small pieces with vitrifiable material, providing a high intensity light source of sufficient power to cause melting and subsequent vitrification of said waste, and cooling and storing of said vitrified material.
U.S. Pat. No. 5,342,582 discloses an apparatus for reprocessing special wastes of photopolymerizable scrap material to produce domestic waste, comprising a housing equipped with a feed hopper, at least one UV emitter arranged in the housing to irradiate and heat the scrap material, and a chopper arranged in the housing to comminute the scrap material. The photocrosslinkable and thermally crosslinkable scrap is composed of, for example, dry resist, solder resist, color proof films, screen printing films, and the like, which form special waste because of their reactive constituents.
U.S. Pat. No. 5,476,975 discloses a method for photodegradation of a solution of organic toxic chemicals recoverable from contaminated wood products by the use of a super-critical fluid by exposing the extracted solution to UV in the presence of a photosensitizer.
U.S. Pat. No. 5,935,525 discloses a pre-treatment system and an air treatment system for abatement of contaminated air that includes pollutants such as VOCs, NOx, and/or carbon monoxide (CO). The air stream is treated using UV light under conditions that produce hydroxyls, peroxides, and other oxidants without the formation of ozone. These oxidants are also used in the activated air with activated water being formed as an aqueous solution (vapor) of the activated air. The pre-treatment system includes a quenching zone where activated water is misted into the air stream, followed by alternating reaction zones and depletion zones where activated air is added and then turbulently mixed with the air stream. The air treatment system includes a primary treatment tunnel, a carbon bed system, an activated air generator, and a sparger tank farm. Activated air produced by the generator is added to water while being exposed to UV light in the sparger tank farm. As the contaminated air stream moves through various sequential chambers within the tunnel, it is subjected to the misted activated water while being simultaneously exposed to UV radiation. Air exiting the tunnel is then further treated in the carbon bed system.
U.S. Pat. No. 6,179,971 discloses a two-step process for air purification comprising a photolytic step followed by a photocatalytic step, each of which entails radiation treatment to convert contaminants into less harmful products. The method provides a photolytic stage having a source of UV radiation and a downstream photocatalytic stage using a photocatalyst and a source of UV radiation.
U.S. Pat. No. 5,538,537 discloses a method of desulfurizing furnace flue gases laden with sulfur dioxide (SO2) comprising cooling the flue gases to a temperature near but above the dew point thereof and flowing the cooled flue gases through a bed of granular cement stone sorbent prepared from a mixture of cement and water. The sorbent laden with pollutants from the flue gases can be further processed directly in an advantageous manner in a cement plant, for example, by grinding it together with cement clinker or separately therefrom and thereby adding it as a component, for example as a gypsum component, to a cement that is to be produced, so that no disposal problems exist for the sorbent laden with pollutants. With the addition of ashes or fly ashes from coal or fluosolids furnaces, a particularly environmentally friendly means for disposal of these ashes can be achieved simultaneously if a sorbent laden with pollutants from the flue gases is further processed for the production of cement (together with cement clinker). Sorbent is produced. It is advantageous to use it with a grain size of greater than 1 mm, preferably approximately 4 to 20 mm. A mixture of granulated cement stone and carbonaceous sorption material then forms the sorbent used according to the invention, which is brought into contact with flue gases that are to be purified.
U.S. Pat. No. 4,634,583 discloses a method for the desulfurization of a calcium-containing flue gas stream from a firing system such as a cement-making plant wherein at least partially deacidified, hot, raw cement meal is added to the flue gas at selected points to adsorb the sulfur oxides onto the calcium present in the gas. No additional adsorption agents, for example, activated carbon, pure calcium oxide, milk of lime, or the like are used. Raw cement meal having an adequately high proportion of free calcium oxide is conveyed to the conduit of the exhaust gas to be desulfurized. The preferred method comprises suspending the deacidified raw cement meal in the flue gas in the form of a cloud of airborne dust and, thereafter, separating the dust from the flue gas after the sulfur oxides have been bonded to the calcium.
U.S. Pat. No. 5,137,704 discloses a process for decreasing the NOx content of exhaust gases from cement-burning kilns by an addition of ammonia and/or ammonia-containing substances to the hot exhaust gases. The exhaust gases are desulfurized at a temperature from 50° C. to 100° C. in a dry or semidry process by a mixture of raw cement powder and calcium hydroxide. The mixed solids that have been removed from the exhaust gas in a dry state in the desulfurizing stage are returned to the exhaust gas stream at temperatures from 850° C. to 1,000° C.
Treatment methods for pollutant-bearing gas in a corona discharge device is a known method of removing the pollutants. A general review of this technique is provided in Puchkarev et al., “Toxic Gas Decomposition by Surface Discharge,” Proceedings of the 1994 International Conf. on Plasma Science, Jun. 6–8, 1994, Santa Fe, N.M., paper No. 1E6, page 88. Corona discharge systems used for removal of mercury are disclosed in U.S. Pat. No. 5,591,412.
Injection of activated carbon in waste gas effluent is known. See U.S. Pat. Nos. 4,196,173; 4,889,698; 5,053,209; 5,607,496; and 5,672,323.